Fatigue tolerant design of steel components based on the size of large inclusions

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Abstract
Fatigue failures in high strength steel components often originate from large, brittle inclusions. The durability of the components is strongly dependent on the size of the inclusions and the magnitude of the local stresses caused by the applied loads. A successful design must consider both the size and the number of large inclusions as well as the stress distribution arising from the geometry and loading of the component. This paper presents a new approach to the safe fatigue design of steel components based on the size distribution of large inclusions in a component with a given stress distribution. The procedure is illustrated using the example of the stress distribution around a hole in a plate, with the size of large inclusions in a large volume of steel estimated by the Generalized Pareto Distribution (GPD) method. It is found that the single largest inclusion is unlikely to lie in a highly stressed volume, but that the more frequently occurring slightly smaller inclusions contribute more to the probability of a fatigue failure. Knowledge of the shape of the size distribution over a range of large sizes, not solely that of the largest size, is therefore essential. The new approach offers a quantitative measure of the improvement in durability to be expected from reduction of the design stress range of a component and from improvements in steel cleanness.

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